What do you do with the last two weeks of physical science class before the final? I needed to come up with a new, fun, and engaging unit that the students would get into, knowing that the material would not be covered on the final exam. I settled on Color Theory. I haven’t taught color in 4 years because it was too abstract a topic and we didn’t have the necessary scaffolding in place to make it concrete. This year was different: I had just built 5 new Color simulations for The Physics Classroom, and I was eager to try them with my students. Armed with the simulations and a suite of new activities, as well as several small mirrors, some colored filters, and an example photograph printed on three separate sheets of plastic in cyan, magenta, and yellow, we spent 7 days exploring the physics and perception of color. Here is the unit:

Day 1 – EM Spectrum Reading & worksheet

I began the unit with three questions:

“What is light?” I asked students to draw a picture or explain this in words. I then put student responses onscreen for the class to discuss.

“Can you hold light in your hand?” I asked students to vote individually, then had them decide as a class.

“What is color?” Is it

Something that exists in your head, or

Something that is real, physical, and measurable?

Students then read “The Electromagnetic Spectrum” from their textbooks and completed the EM Spectrum Worksheet. Students showed me the completed worksheet before the bell rang.

As an extra credit assignment, I told the students to create their own mnemonic to memorize the EM spectrum. An example mnemonic: radio waves, microwaves, infrared, visible, ultraviolet, xray, gamma -> “Red Martians Invade Venus Using Xray Guns”. In order to receive credit, students had to repeat their own mnemonic and what it meant from memory.

Day 2 – “Rippin the Rainbow a New One”

I explained to students that they were learning about the nature of color by listening.

This was a great day for discussion – students loved talking about the mantis shrimp!

Day 3 – Red, green, and blue light can be combined to create yellow, cyan, magenta, white, or any other color.

I projected the “RGB Color Addition” simulation onscreen. We practiced mixing the colors in the simulation.

Taking advantaged of the colored beams from the projector, I used two mirrors to redirect the red and green beams onto a white wall to demonstrate that red and green make yellow. The picture above shows the setup for this.

I then repeated the mirror trick with different color values to make every color of the rainbow. With 3 mirrors shining red, green, and blue light on the wall, students were able to make a convincing white.

Students then loaded the RGB Color Addition simulation and completed the “Color Addition Lab”. They had to show me the completed worksheet before the bell rang.

Day 4 – Colored Shadows

We started by reinforcing the color combinations from Day 3. I used the RGB Color Addition simulation to mix blue and green to make cyan. Then I used one mirror to shine cyan light and a different mirror to shine red led onto a white wall. We showed that mixing cyan and red makes white.

We repeated this process for yellow + blue and magenta + green, always making white.

I then showed the “RGB Lighting” simulation. I gave students minimal instruction, and had them use the simulation to complete the “Color Shadows Worksheet”. They had to show me the completed worksheet before the bell rang.

Day 5 – Colored Filters

Today is the day when things started getting difficult for the students. They easily understood that a single filter blocks light, but things got tougher when they had to use combinations of filters to create different RGB values.

I used the RGB Color Addition simulation to display white light onscreen. I held up a cyan filter in one hand and a magenta filter in the other hand. I then overlapped the cyan and magenta filters to get blue. This was not much of a surprise for the students.

I then held a red filter over a blue filter. This did not make purple, it made black! These are different from what we learned on the previous few days; filters work by removing colors, not by adding them. I had access to RGB and CMY filters, so I was able to show the following combinations:

Red + green = black

Blue + green = black

Blue + yellow = black

Cyan + red = black

Magenta + green = black.

Students worked with the “Colored Filters” simulation, completing the corresponding lab activity sheet. They had to show me the completed worksheet before the bell rang

Day 6 – CMY Colored Pigments (Color by Subtraction)

I started by showing students cmyk print test pages on transparent film. You can make your own by splitting a photograph into its yellow, magenta, and cyan components, and then printing each color onto separate transparencies. By laying down magenta + yellow, we can make all shades of magenta, red, orange, yellow, etc. By adding a layer of cyan, all colors of the rainbow can be made. I had students experiment with the C, M, and Y sheets. I also showed them that black is just added to make the colors “pop” with more contrast – we can still make black without black ink.

I explained that pigment works the same way as filters. Mixing blue and yellow doesn’t make white, it makes (almost) black. Why is this? After a few minutes of students hypothesizing, I explained that paint works the exact same way as filters – it removes color. Red paint absorbs green and blue, reflecting only the available red light. Green paint absorbs red and blue, reflecting only the available green light, etc. Yellow pint absorbs blue, reflecting both red and green.

This means that the primary colors for pigment are Cyan, Magenta, and Yellow! I then showed students an ink cartridge, and pointed out the “C”, “M”, “Y”, and “K” ink tanks.

We started the “Painting with CMY” activity sheet together. I explained that colored paper doesn’t actually create its own colored light, it merely reflects whatever light that is shined on it that it doesn’t absorb. This can be represented by the following diagrams:

Example of Reflection and Absorption for Blue and Magenta Paper

Finally, students loaded the “Painting with CMY” simulation and did the activity sheet. They had to show me the completed worksheet before the bell rang

Day 7 – What color would a _______ object appear under _______ light?

This was the “Test” day for students. They would use their understanding of color reflection, absorption, and perception to fully explain different situations.

I started by showing students how to use the “Stage Lighting” sim. In particular, I showed them that when you shine magenta lights on a yellow banana, the banana appears red. The picture below demonstrates this apparently discrepant event:

Magenta Light on a Yellow Banana

I then split students into groups of 2. Each group use the Educreations app on their ipad to explain why __________ paper appears ___________ under ___________ light. Each group had to explain one of the following combinations:

magenta light + cyan paper = you see paper as blue

magenta light + yellow paper = you see paper as red

yellow light + cyan paper = you see paper as green

yellow light + magenta paper = you see paper as red

cyan light + yellow paper = you see paper as green

cyan light + magenta paper = you see paper as blue

Their responses had to include a diagram showing absorption and reflection. I told them to look back at their “CMY Colored Pigments” lab to help with this.

They could use the last simulation, “Stage Lighting“, as well as all of the other simulations, plus all of their existing notes and worksheets.

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